Field of the Invention
The invention relates generally to electric motors, and more particularly to systems and methods for reducing or preventing degradation of electrical insulation that separates the windings of a motor for use in such applications as submersible electric pump (ESP) systems by providing a hermetic seal that prevents the ingress of wellbore fluids into the motor's stator chamber where they can reach the windings.
Related Art
Artificial lift systems such as ESPs are commonly used in the production of oil from wells when the pressure in a producing geological formation is not sufficient to force the oil out of the formation and to the surface of the well. The ESP is positioned in the well, at or near the producing formation, and is operated to pump the oil out of the well.
The ESP includes a pump and an electric motor. The motor is typically coupled to an electric drive system positioned at the surface of the well. The drive provides power to the motor to run the motor. The motor in turn drives the pump, which lifts the oil out of the well. The motor may have a rotary design in which a rotor rotates within a stator, or it may have a linear design in which a mover moves in a reciprocating motion within the stator. The present disclosure will focus on rotary designs, but may be applicable to linear designs as well.
The ESP motor uses windings or coils of electrical wire in a stator to generate rotating magnetic fields. These rotating magnetic fields interact with the magnetic fields of the rotor, causing it to move within the motor. The magnetic fields of the rotor may be produced by permanent magnets in the rotor (in a permanent magnet motor), or they may be produced by electrical currents that are induced in the rotor by rotor slip with respect to the magnetic fields of the stator (in an induction motor).
The windings in the stator normally have a layer of electrical insulation that electrically isolates the windings' conductors. If wellbore fluids leak into the motor and come into contact with the electrical insulation, they can cause or accelerate degradation of the insulation. Even if the well fluid does not get into the motor, the winding can also be adversely affected by the self-contained moisture in the motor oil. In some cases, dielectric oil which is in the motor may contain additives for lubrication purpose, such as esters that can react with moisture or water in the motor oil at elevated temperatures to produce alcohols and acids. These may further accelerate degradation of the electrical insulation. Eventually, degradation of the windings' electrical insulation may be substantial enough to result in short circuits between turns of the windings, between different phases (i.e., between separate windings that are energized by different phases of the input power), and even between one of the phases and ground, any of which may reduce the efficiency of the motor or cause it to fail.